CHEMOTAXIS OF OXYTRICHA .AERUGINOSA. 295 



one or both the unknown substances attracted the organism, 

 while the acid above a certain strength repelled it. The Oxytricha 

 under the influence of these opposite forces, was held along a line 

 where the two balanced each other. According to Jennings, an 

 exactly similar phenomenon occurs in Paramecium, when exposed 

 to a mixed solution of acid and table salt. 



It appears then that we have here at least two unknown sub- 

 stances, one volatile, the other not, affecting the chemotactic 

 reactions of Oxytricha. The presence of two such substances, 

 whatever may be their origin, helps to explain away a difficulty 

 which I think has been overlooked. In the ring formation so 

 commonly occurring, the Oxytricha ave at first distributed equally 

 around the circle. They are held within it, so we suppose, because 

 some substance is there in optimum quantity. As the drop, say 

 of distilled water, diffuses this optimum area moves outward, and 

 the organisms move with it. However, if undisturbed, the 

 Oxytricha will soon be congregating in some one portion of the 

 circle and forming a dense mass, entirely similar to the ordinary 

 spontaneous aggregations. Now on the supposition that there is 

 only one attractive substance concerned, such aggregation in one 

 part of a circle which is supposed to uniformly contain an opti- 

 mum amount of this attractive substance is hard to understand. 

 But if we suppose that in the dilution of one of two attractive 

 substances to an optimum strength the second substance is 

 reduced below its' optimum, then it is easily seen that the meet- 

 ing of two or three Oxytricha in some part of the circle would 

 soon tend to bring this second substance to its optimum strength 

 in that part of the circle. Then naturally any further individuals 

 entering this smaller area would be retained there, in similar 

 manner to that by which they are retained in the circle as a 

 whole. 



I can only add that there is some evidence that these sub- 

 stances are formed with the development of a colony, and that the 

 character of the chemotactic reaction itself is altered in the course 

 of such development. A culture of Oxytricha rejuvenated by 

 addition of fresh hay and water, acted very anomalously. The 

 creatures were positive to distilled water, as well as to n/ioo HC1. 

 But two days later all this had changed, and the reactions to both 

 substances were negative. 



